This invention relates to insulating foams, and more particularly to foams having unusually low thermal conductivity.
Insulating foam, particularly polyurethane foam, is an indispensable constituent of many refrigerators and similar cooling units. By reason of increasingly strict Federal Government energy conservation standards, it is of interest to develop foams having substantially lower thermal conductivity than those presently available.
Conventional insulating polyurethane foam is generally prepared by the reaction of at least one polyol with at least one diisocyanate in the presence of suitable catalysts, surfactants and blowing agents. Among the blowing agents employed are halogenated compounds, which may be halocarbons such as monofluorotrichloromethane and/or halohydrocarbons such as 1,1-dichloro-1-fluoroethane. They remain in the cell voids of the foam after blowing, and their presence therein is beneficial since they have low thermal conductivity and thus contribute to the insulating quality of the foam.
Also usually present in the reaction mixture is water, which serves at least three purposes. First, in small quantities it is effective to improve the flow properties of the mixture. Second, it reacts with a minor proportion of the diisocyanate to form carbon dioxide, which is also effective as a blowing agent. Third, it reacts to form substituted urea crosslinking moieties, thus stabilizing the foam as it is produced.
The use of many halocarbons is now severely restricted and will soon be prohibited by reason of their property of depleting the stratospheric ozone layer. Halohydrocarbons are an interim solution but their use will ultimately also be prohibited. It is therefore necessary to develop methods for blowing insulating foam which do not require the use of halogen-containing blowing agents. At the same time, however, a high degree of insulating power is more necessary than ever to decrease energy consumption. An additional desirable factor is suppression of the "greenhouse effect" which promotes global warming.
Carbon dioxide has a high thermal conductivity, on the order of 17 milliwatts/m-.degree. K. Thus, a foam prepared with the use of a blowing agent combination including 15% and 50% (by volume) carbon dioxide with the balance being monofluorotrichloromethane has a thermal conductivity approximately 5% and 15%, respectively, higher than a corresponding foam prepared without the use of carbon dioxide.
It would thus be of considerable interest to devise methods for producing insulating foam which employ carbon dioxide as a blowing agent, but which also produce a foam free from carbon dioxide or containing only very minor proportions thereof. It would further be of interest to incorporate in the foam cells a gas of very low thermal conductivity, reducing the conductivity of the foam itself to a value roughly equal to that obtained by the use of monofluorotrichloromethane as a blowing agent.
It is known to incorporate gases of low thermal conductivity, particularly noble gases such as krypton, in insulating panels at atmospheric pressure. However, krypton is very expensive; its use in foam at atmospheric pressure adds about one dollar per board foot to the cost of the insulation. Xenon, another noble gas with potential for such use, is even more expensive, adding $4.50 per board foot at atmospheric pressure. Therefore, the production of foams of this type has been economically unfeasible.
The present invention provides an economically feasible method for producing plastic foams blown with carbon dioxide in combination with an inert gas, and subsequently removing said carbon dioxide to produce an insulating foam which is very low in thermal conductivity and which is in other respects environmentally beneficial or neutral. Said method is adapted to in situ production of carbon dioxide in conventional equipment used for the production of polyurethane foam and the like. Also provided are resinous foams of unusually low thermal conductivity.